This invention relates to a film tube having a substrate which has a low wettability, for example polyolefine plastic film, or the like, such that printing ink or adhesive cannot be readily applied thereon, unless it is treated to increase wettability. This invention also relates to a method for forming the film tube by sealing the same by overlapping side edges one another.
A conventional method for successively forming a film tube which is generally used as a tubular label for packaging articles, involves rolling up a continuous web of a film. One surface of the film has a printing ink layer. The film is then unwound and, simultaneously, opposite longitudinal side edges of the film are overlapped. The overlapping side edges of the film are bonded or heat-sealed together.
Polyethylene film, polypropylene film, or the like, in the form of a heat-shrinkable film or stretch film, is generally used as a material for packaging articles and a tubular label for displaying an article's name or the like thereon. The shrinking force of the heat-shrinkable film or the elastic stretching stress of the stretch film allows the film to tightly conform around the articles.
However, since polyolefine plastic film, including polyethylene film and polypropylene film, inherently have a low degree of wettability, printing ink, adhesive or the like cannot be readily applied thereon. Wettability can be increased by activating a surface of the film. It is subjected to surface treatment, typically corona electrical discharge treatment, prior to applying the printing ink or the adhesive thereon. The wettability of the activated surface is preferably more than 37 dyn/cm, which is determined by the JIS (Japanese Industrial Standard) K6768 testing method. Thus, the activated surface has increased adhesive strength.
In this method, only one surface of the film is subjected to corona electrical discharge treatment to avoid blocking (coalescence) which may occur in a roll of a continuous web of the film when both surfaces of the film are subjected to corona electrical discharge treatment. The blocking may damage the film.
However, it is conceivable that when only one surface of the film is subjected to corona electrical discharge treatment, the other surface of the film will still have low wettability. That is, it is difficult to obtain sufficient adhesive strength on the longitudinal side edges of the film, opposite the side edges subjected to surface treatment, after the side edges of the film are bonded with one another by adhesive. Such poor adhesive strength may cause undesirable separation of the overlapping side edges of the film from one another. To overcome this drawback, the width of the overlapping side edges may be enlarged, or the overlapping side edges; may be heat-sealed to one another to obtain sufficient adhesive strength.
However, enlarging the film may result in an increase in the cost of the film. In addition, it is difficult to obtain sufficient adhesive strength due to the fact that when the film tube, formed from the stretch film, is forcibly stretched to tightly conform around articles by using the elastic stretching stress of the stretch film as a tightening force, the elastic stretching stress is applied onto the bonded side edges of the film as shearing stress, which may readily cause separation of the bonded side edges of the film. Further, it is difficult to maintain the bond strength of the side edges of the film long enough to tightly package the articles for a prolonged period of time.
In a heat sealing method, it is possible to obtain sufficient-bond strength in the overlapping region of the film by properly controlling the temperature for the heat sealing. However, when using a film having a low melting point, such as polyethylene film, as a substrate of the film tube, it is difficult to control the temperature for heat sealing. As a result, the overlapping side edges of the film may be unsecurely sealed with one another. Accordingly, it is difficult to successively form the film tube effectively.